Self-aligning decoupled nut mechanism

ABSTRACT

A feed screw and nut mechanism includes a nut having an internally threaded bore which threadingly engages an externally threaded feed screw. A runner bearing is attached to the nut and projects outwardly therefrom, and a dummy runner bearing is attached to the nut and projects outwardly therefrom and is angularly displaced from the runner bearing about the feed screw axis. The mechanism includes a stationary runner defining a runner surface that is engaged by the runner bearing to prevent rotation of the nut, and a dummy runner defining a dummy runner surface that engages the dummy runner bearing. The dummy runner and dummy runner bearing are biased toward each other such that the nut is rotatably biased in a direction to urge the runner bearing against the runner surface. The nut includes at least one rotatable drive bearing having a rotation axis perpendicular to the feed screw axis, and at least one rotatable driven bearing connected to the carriage and having a rotation axis perpendicular to both the feed screw axis and to the axis of the drive bearing. The drive and driven bearings make contact at their cylindrical surfaces to form a crossed bearing coupling for minimizing transmission to a carriage of force components which are not parallel to the axial direction along which the carriage travels. In a preferred embodiment, the nut mechanism includes a slave carriage, connected to the carriage, which supports a pair of crossed bearing couplings and which is rotatably connected to the nut to isolate the slave carraige from rotation of the nut about a transverse axis.

FIELD OF THE INVENTION

[0001] The present invention relates to a nut mechanism for creatingtranslational motion by engagement with a rotating threaded drive shaftor feed screw.

BACKGROUND OF THE INVENTION

[0002] In a variety of mechanical devices and systems, feed screw andnut arrangements are frequently used for providing translational motionto a moving carriage or the like. Typically, the nut is rigidly affixedto the carriage and is prevented from rotating about its axis, such thatrotation of the feed screw causes the nut, and hence the carriage, totranslate along the feed screw axis.

[0003] In some applications, precision of movement of the carriage isnot of particular concern, and hence factors such as dimensionalimperfections and friction which are common or inherent in feedscrew/nut arrangements are relatively insignificant problems. However,in other applications requiring precise movement and positioning of acarriage, these factors can be significant problems. For example, smallerrors in the manufacture of the various components of a feed screw/nutarrangement can lead to various imperfections including misalignment ofthe feed screw axis with the direction of carriage movement, slighteccentricity of the nut with respect to the feed screw, and otherproblems. These dimensional imperfections can result in forces beingexerted on the carriage in directions other than the intended directionof movement, which can cause deflection of the carriage or otherundesirable consequences. Furthermore, in all feed screw/nutarrangements, there is friction between the rotating feed screw and thenut. In most conventional feed screw/nut arrangements wherein the nut isrigidly connected to the carriage, frictional forces exerted on the nutare transmitted to the carriage, which again can cause deflection of thecarriage and other problems.

[0004] Because of problems such as those mentioned above, efforts havebeen made toward developing feed screw/nut arrangements havingself-aligning nuts which can tolerate a certain amount of misalignmentbetween the feed screw and the carriage or other structure on which thenut is carried. For example, U.S. Pat. No. 3,977,269 discloses aself-aligning nut mechanism having an elongate tubular nut body formedof a base portion and an internally threaded portion connected thereto.The internally threaded portion has a plurality of longitudinal slotsthat divide it into multiple elements whose thread formations areadapted for contact with the screw threads. The base portion of the bodyhas a pair of oppositely disposed transverse slots enabling the threadedelements to have limited movement in radial directions with respect tothe screw axis. Yieldable means are carried by the threaded elements forbiasing them toward one another so as to maintain an intimate engagementof the threaded formations with the screw threads. An elongate springsleeve surrounds and is concentric with the nut body, and has threepairs of oppositely disposed transverse slots spaced apart along thelength of the sleeve. The three pairs of slots are indexed 90° withrespect to one another to enable pivotal movement of the adjacent sleeveparts. One extreme sleeve part carries the nut body and the otherextreme sleeve part is adapted to be secured to a reciprocating part ofa machine.

[0005] The self-aligning nut of the '269 patent thus purports to addressthe problem of misalignment between a feed screw and the reciprocatingmachine part, by allowing the nut to resiliently conform to the feedscrew and maintain threaded engagement therebetween, and by allowing thesleeve parts to move relative to each other to compensate for slightmisalignment between the screw axis and the machine part. However, the'269 patent does not purport to address the problem of forces beingtransmitted to the machine part in directions other than the intendeddirection of movement of the machine part. Even though the nut andsleeve arrangement purportedly compensates for misalignment and remainsengaged with the feed screw, such misalignment would result in forces onthe nut and/or on the machine part in directions other than the intendeddirection of movement, which forces are undesirable where highly precisemovement of the machine part is required. Additionally, frictionalforces between the feed screw and the nut would also result in suchundesirable forces.

SUMMARY OF THE INVENTION

[0006] The aforementioned problems are overcome and other advantages arerealized by a nut mechanism in accordance with the present invention. Inaccordance with one preferred embodiment of the invention, a nutmechanism for translating a carriage along an X-axis includes anexternally threaded feed screw which is rotatable about a fixed screwaxis parallel to the X-axis, and a nut having an internally threadedbore which threadingly receives the feed screw. The nut mechanismfurther includes a stationary runner adapted to be fixed relative to thefeed screw axis, the stationary runner defining a stationary runnersurface which extends parallel to the X-axis, and a runner bearingattached to the nut and projecting outwardly therefrom along a Y-axiswhich is perpendicular to the X-axis. The runner bearing engages thestationary runner surface to prevent rotation of the nut when the feedscrew is rotated such that rotation of the feed screw causes the nut totranslate along the X-axis and the runner bearing to travel along thestationary runner surface. The mechanism also includes a dummy runnerbearing attached to the nut and projecting outwardly therefrom along anaxis which is perpendicular to the X-axis and angularly displaced aboutthe screw axis from the runner bearing, and a dummy runner defining adummy runner surface which extends parallel to the X-axis and engagesthe dummy runner bearing. The dummy runner bearing and dummy runner arebiased toward each other so as to rotatably bias the nut in a directionto maintain the runner bearing in contact with the stationary runnersurface. Thus, the dummy runner bearing and dummy runner ensurecontinuous contact of the runner bearing with the stationary runnersurface such that rotation of the feed screw tends to cause translationof the nut rather than rotation of the nut with the feed screw.

[0007] Preferably, the nut mechanism includes bearing elements whichsubstantially prevent the nut from transmitting forces to the carriagein directions non-parallel to the X-axis along which the carriage moves.Specifically, the nut mechanism in a preferred embodiment includes adrive bearing attached to the nut and projecting outwardly therefrom,the drive bearing having an outer generally cylindrical drive surfacedefining an axis which is perpendicular to the X-axis; and a drivenbearing which has an outer generally cylindrical driven surface definingan axis and which is adapted to be attached to the carriage such thatthe axis of the driven surface is perpendicular to both the axis of thedrive surface and the X-axis, and such that the driven surface isengaged by the drive surface to form a crossed bearing coupling. Thedrive and driven bearings advantageously are freely rotatable abouttheir axes. The crossed bearing coupling formed by the cylindricalsurfaces of the drive and driven bearings perpendicularly orientedrelative to each other approximates a frictionless contact between asphere and a flat surface wherein only forces normal to the flat surfacecan be transmitted to the surface by the sphere. Thus, any erroneousmotions of the nut which would otherwise result in forces on thecarriage in directions non-parallel to the X-axis will instead result inrotation of one or both of the drive and driven bearings about theiraxes, and accordingly the force transmitted from the drive bearing tothe driven bearing is substantially entirely in a direction parallel tothe X-axis. Nonaxial forces on the carriage are thereby minimized.

[0008] In accordance with another preferred embodiment of the invention,the nut mechanism includes a slave carriage connected with the nut andadapted to engage the carriage for transmitting force in the X-axisdirection from the nut to the carriage while isolating the carriage fromrotational motion of the nut about the Y-axis. The slave carriage isconnected to the nut so as to be rotatable relative to the nut about theY-axis which defines the axis of the runner bearing. Advantageously, apair of bearings are mounted on opposite sides of the nut and connectedto the slave carriage for rotatably connecting the slave carriage to thenut. Accordingly, erroneous rotational motion of the nut about theY-axis will tend to be taken up by the rotatable connection with theslave carriage so that such motion of the nut does not result innonaxial forces being transmitted to the carriage.

[0009] Preferably, the slave carriage mounts at least one drive bearingas described above. More preferably, a pair of drive bearings aremounted on one side of the slave carriage and another pair of drivebearings are mounted on an opposite side of the slave carriage. Thedrive bearings of each pair are spaced apart in the X-axis direction,and a driven bearing is disposed between the spaced-apart drive bearingsto form a biaxial crossed bearing coupling permitting forces to betransmitted to the carriage in two opposite directions along the X-axis.The two biaxial crossed bearing couplings are preferably symmetricallydisposed about the screw axis such that the driven bearings cooperate toexert axial force on the carriage along a line of action which iscollinear with the screw axis.

[0010] In accordance with still another preferred embodiment of theinvention, a nut mechanism is provided having a frame defining aninterior space therein and having an opening for passage of the feedscrew into the interior space; a stationary runner within the interiorspace and connected to the frame so as to be fixed relative to the screwaxis, opposite sides of the stationary runner respectively defining astationary runner surface which extends parallel to the X-axis and adummy runner surface which is spaced apart from and parallel to thestationary runner surface; a nut disposed in the interior space andhaving an internally threaded bore adapted to threadingly receive thefeed screw, the nut including a portion which is resiliently bendablerelative to the remainder of the nut in a plane perpendicular to theX-axis; a runner bearing attached to the nut and projecting outwardlytherefrom along a first axis which is perpendicular to the X-axis, therunner bearing engaging the stationary runner surface to preventrotation of the nut when the feed screw is rotated such that rotation ofthe feed screw causes the nut to translate along the X-axis and therunner bearing to travel along the stationary runner surface; and adummy runner bearing attached to the resiliently bendable portion of thenut and projecting outwardly therefrom along a second axis which isparallel to the first axis of the runner bearing and spaced aparttherefrom, the dummy runner bearing engaging the dummy runner surface.The resiliently bendable portion of the nut preloads the dummy runnerbearing against the dummy runner surface so as to rotatably bias the nutin a direction to maintain the runner bearing in contact with thestationary runner surface. As a result, rotation of the feed screwcauses the nut and, in turn, the carriage to translate along the X-axisdirection.

[0011] The invention thus provides unique nut mechanisms having featuresfor minimizing nonaxial forces exerted on a moving carriage caused byfeed screw and/or nut misalignment, dimensional imperfections ofcomponents, and friction. In addition, the nut mechanism of the presentinvention preferably maintains the runner bearing in contact with thestationary runner surface such that rotation of the feed screw will notcause the nut to rotate due to frictional forces between the nut andfeed screw, but instead will cause the nut to be translated along thefeed screw so as to move the carriage in the X-axis direction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above and other objects, features, and advantages of theinvention will become more apparent from the following description ofcertain preferred embodiments thereof, when taken in conjunction withthe accompanying drawings in which:

[0013]FIG. 1 is a perspective view of a nut mechanism in accordance witha first preferred embodiment of the invention;

[0014]FIG. 2 is a perspective view of a feed screw and a nut inaccordance with a second preferred embodiment of the invention;

[0015]FIG. 3 is a cross-sectional view of the nut of FIG. 2 and alsoshowing a slave carriage assembly connected to the nut;

[0016]FIG. 4 is a perspective view of a complete nut mechanism inaccordance with the second preferred embodiment of the invention,partially broken away to show the nut and slave carriage assembly;

[0017]FIG. 5 is a side elevational view of the nut mechanism of FIG. 4;

[0018]FIG. 6 is a cross-sectional view of an external connector forconnecting the slave carriage to a carriage;

[0019]FIG. 7 is a perspective view of the mechanism of FIG. 4, alsoshowing the external connector connected to the slave carriage; and

[0020]FIG. 8 is a cross-sectional view of a nut mechanism in accordancewith a third preferred embodiment of the invention in which the nutincludes a resiliently bendable portion for biasing the dummy runnerbearing against the dummy runner surface.

DETAILED DESCRIPTION OF THE DRAWINGS

[0021] The invention is now explained by reference to certain preferredembodiments thereof. It will be understood, however, that the inventionis not limited to these embodiments, but can take a variety of otherforms within the scope of the appended claims.

[0022] With reference to FIG. 1, a nut mechanism in accordance with afirst preferred embodiment of the invention is broadly indicated byreference numeral 20. The nut mechanism 20 includes a feed screw 22comprising an externally threaded shaft or rod oriented with its axis 24nominally aligned along the direction of an X-axis 26 which defines thedirection along which a carriage C is to be translated. The nutmechanism 20 further includes a nut 28 which has an internally threadedbore 30 formed therethrough and through which the feed screw 22 isthreadingly received. The mechanism 20 also includes a stationary runner32 which is fixed relative to the feed screw axis 24 and which defines astationary runner surface 34 that extends parallel to the X-axis and isspaced from the nut 28.

[0023] The nut 28 includes a runner bearing 36 having a shaft 38attached to the nut and projecting generally radially outward therefromalong a Y-axis 40 which is perpendicular to the X-axis. The runnerbearing 36 has an outer generally cylindrical surface 42 which iscoaxial with the Y-axis. The outer surface 42 of the runner bearing 36contacts the stationary runner surface 34. Thus, clockwise rotation ofthe feed screw 22 (as viewed in the positive X-axis direction) causesthe nut 28 to tend to also rotate clockwise because of friction betweenthe feed screw and nut, but engagement of the runner bearing 36 with thestationary runner surface 34 prevents such rotation of the nut.Accordingly, the nut 28 is translated in the X-axis direction along theaxis 24 of the feed screw, and the runner bearing 36 rolls along thestationary runner surface 34.

[0024] To maintain the runner bearing 36 in contact with the stationaryrunner surface 34 such that rotation of the feed screw 22 does notresult in rotation of the nut 28 about its axis due to friction betweenthe feed screw and nut, the nut mechanism 20 includes a dummy runnerbearing 44 having a shaft 46 attached to the nut 28 and projectinggenerally radially outward therefrom, and a dummy runner 48 defining adummy runner surface 50 that extends parallel to the screw axis 24 andcontacts the dummy runner bearing 44. The dummy runner 48 and dummyrunner bearing 44 are biased toward each other. The dummy runner bearing44 is angularly displaced from the runner bearing 36 such that in acircumferential direction the runner bearing 36 and dummy runner bearing44 are between the stationary runner surface 34 and dummy runner surface50. Thus, the biasing force between the dummy runner bearing 44 anddummy runner surface 50 causes the nut to be rotatably biased in adirection to urge the runner bearing 36 against the stationary runnersurface 34. In the embodiment illustrated in FIG. 1, the dummy runnerbearing 44 and dummy runner surface 50 are diametrically opposite therunner bearing 36 and stationary runner surface 34, but it is notcritical that they be diametrically opposite, and other angulardisplacements of less than 180° are also possible. The dummy runner 48advantageously is movably mounted relative to the screw axis 24 andurged by resilient elements (not shown) toward the dummy runner bearing44. However, it will be recognized that equivalently the dummy runner 48can be fixed relative to the screw axis 24 and the dummy runner bearing44 can provide the spring force. For example, the shaft 46 of the dummyrunner bearing 44 can be made resiliently bendable for urging the dummyrunner bearing 44 against the dummy runner surface 50.

[0025] The nut mechanism 20 also includes features for transmittingforce to the carriage C in the axial direction of the X-axis whileminimizing the transmission of nonaxial force components that are notparallel to the X-axis. Specifically, the mechanism 20 includes acrossed bearing coupling 52 comprising a drive bearing 54 connected tothe nut 28 and a driven bearing 56 connected to the carriage C. Thedrive bearing 54 is connected to the nut 28 by a shaft 58 which projectsgenerally radially outward from the nut along a Z-axis 60 which isperpendicular to the X-axis and to the Y-axis. The drive bearing 54 isrotatable about the Z-axis and includes an outer generally cylindricalsurface 62 which is coaxial with the Z-axis. The driven bearing 56 isrotatably mounted on the carriage C such that it is rotatable about anaxis 64 which is perpendicular both to the X-axis and to the Z-axis ofthe drive bearing 54. The driven bearing 56 has an outer generallycylindrical surface 66 which is in contact with the cylindrical surface62 of the drive bearing 54. Thus, the crossed cylindrical surfaces 62and 66 which are freely rotatable about their respective axes form anapproximation of a frictionless contact between a sphere and a flatsurface wherein only force that is normal to the flat surface can betransmitted from the sphere to the flat surface. In equivalent fashion,if the drive bearing 54 and driven bearings 56 were ideal bearings whichcould rotate without friction, then as long as the Z-axis of the drivebearing 54 remains perpendicular to the X-axis, the drive bearing 54could only transmit force to the driven bearing 56 along a normal to thecylindrical surface 66 in the X-axis direction. Rotation of the nut 28about the X-axis and/or the Z-axis would cause the drive bearing 54 torotate about its axis so that force would continue to be exerted on thedriven bearing 56 only in the X-axis direction. Similarly, translationof the nut 28 in the Y-axis and/or Z-axis directions would cause thedrive bearing 54 and/or the driven bearing 56 to rotate about its axisso that force would continue to be exerted on the driven bearing 56 onlyin the X-axis direction. As a result, the nut mechanism of the presentinvention minimizes deflection of the carriage and other undesirableconsequences caused by the exertion of nonaxial forces on the carriageC.

[0026] In reality, there will be some friction within the drive anddriven bearings, and the frictional forces within the driven bearing 56during rotation thereof will result in a torque being applied to thecarriage C about the axis 64 of the driven bearing. However, this torquewill exist only when the driven bearing is actually rotating and willtypically be very small in relation to the axial force exerted on thecarriage. Thus, the crossed bearing coupling 52 closely approximates aperfect point contact capable of transmitting purely normal force to thecarriage in the X-axis direction.

[0027] It will be recognized that if the nut 28 rotates about theY-axis, the contact between the drive and driven bearing surfaces willshift such that the normal to the driven bearing surface 66 will nolonger be parallel to the X-axis, but will have some component in theZ-axis direction. Accordingly, the force exerted on the driven bearing56 by the drive bearing 54 will have some Z-axis component. However, forsmall angle rotations of the nut about the Y-axis, this Z-axis forcecomponent will be very small in relation to the X-axis force component.It will also be noted that the force exerted on the carriage C by thedrive bearing 54 is not collinear with the screw axis 24, and thereforethe reaction force transmitted back through the nut mechanism will exerta bending moment on the feed screw 22, which may be undesirable in someapplications.

[0028] FIGS. 2-7 depict a second preferred embodiment of the inventionwhich provides a nut mechanism having features for reducing Z-axis forcecomponents on the carriage caused by rotation of the nut about theY-axis, and also having features which ensure that the force exerted onthe carriage has a line of action that is substantially collinear withthe feed screw axis. Thus, with reference to FIG. 4, a nut mechanism 70includes a feed screw 72 having an axis 74 oriented along an X-axis 76which defines the direction of movement of a carriage (not shown) to betranslated by the nut mechanism. The mechanism 70 further includes a nutassembly 78 which engages the feed screw 72 and is translated along theX-axis by rotation of the feed screw.

[0029] As best seen in FIGS. 2 and 3, the nut assembly 78 comprises anut 80 and a slave carriage 82 which surrounds the nut 80. The nut 80includes a runner bearing 84 mounted on a shaft 86 attached to the nut,and a dummy runner bearing 88 mounted on a shaft 90 attached to the nutand diametrically opposite the runner bearing 84, similar to the nutmechanism 20 of FIG. 1. Mounted on the runner bearing shaft 86 is arotatable slave carriage bearing 92, and mounted on the dummy runnerbearing shaft 90 is another rotatable slave carriage bearing 94. Theslave carriage 82 includes bearing holes 96 in its two opposite sidesfor receiving the slave carriage bearings 92 and 94. The slave carriage82 is thus rotatable about the Y-axis relative to the nut 80. The insideheight of the slave carriage 82 in the Z-axis direction is greater thanthat of the nut 80 such that there are gaps 98 between the slavecarriage 82 and the nut 80, permitting the nut to rotate about theY-axis relative to the slave carriage. Therefore, the nut 80 is capableof rotating about the Y-axis without causing the slave carriage 82 tosimilarly rotate about the Y-axis, and accordingly, such rotationalmotions of the nut 80 will tend not to cause Z-axis force components tobe exerted on the carriage which is connected to the slave carriage 82as described below.

[0030] The slave carriage 82 includes features for transmitting axialforce to a carriage. More particularly, the slave carriage 82 includes apair of drive bearings 100 mounted on a wall 102 that is normal to theZ-axis, and another pair of drive bearings 100 mounted on a wall 104 onan opposite side of the nut 80 from the wall 102 and parallel to thewall 102. The two drive bearings 100 of each pair are spaced apart inthe X-axis direction. The drive bearings 100 are rotatable about axes106 that are parallel to the Y-axis. The drive bearings 100 on the onewall 102 and the drive bearings 100 on the other wall 104 are spacedequidistant from the feed screw axis 74 on diametrically opposite sidesthereof. Thus, the drive bearings 100 are symmetric about the feed screwaxis 74 in the XZ plane, so that the net axial force transmitted to acarriage by the drive bearings 100 will be collinear with the feed screwaxis, thereby avoiding the imposition of a bending moment on the feedscrew.

[0031] The nut mechanism 70 also includes an external connector 108 asshown in FIGS. 6 and 7 for facilitating connection between the nutassembly 78 and a carriage. The external connector 108 comprises afour-walled open rectangular structure having a pair of opposite walls110 and 112 which are spaced apart and parallel and are connected toeach other at their ends by a pair of walls 114 and 116 which are spacedapart and parallel to each other. The wall 110 supports a driven bearing118 disposed within the connector 108, and the opposite wall 112supports a driven bearing 120 disposed within the connector 108 andfacing the driven bearing 118. The driven bearings 118 and 120 arerotatable about axes 122 and 124, respectively, which are normal to therespective walls 110 and 112. The external connector 108 mounts aroundthe slave carriage 82 with the wall 110 of the connector confronting thewall 102 of the slave carriage, and the opposite wall 112 of theconnector confronting the corresponding opposite wall 104 of the slavecarriage. The driven bearing 118 resides between the two drive bearings100 on the wall 102, and the other driven bearing 120 resides betweenthe two drive bearings 100 on the opposite wall 104 of the slavecarriage. When the connector 108 is connected to the slave carriage 82,the axes 122 and 124 of the driven bearings 118 and 120 areperpendicular to the X-axis and to the axes 106 of the drive bearings100. Thus, the driven bearing 118 and the corresponding pair of drivebearings 100 form a first crossed bearing coupling, and the drivenbearing 120 and the corresponding pair of drive bearings 100 form asecond crossed bearing coupling. The external connector 108 is adaptedto be connected to a carriage for transmitting force from the nutmechanism 70 to the carriage.

[0032] The crossed bearing couplings formed by the drive bearings 100and associated driven bearings 118 and 120 comprise biaxial couplingsbecause the drive bearings 100 are capable of transmitting force to thedriven bearings 118, 120 either in the positive X-axis direction or inthe negative X-axis direction. As for the crossed bearing coupling 52described above in connection with FIG. 1, the crossed bearing couplingsof the nut mechanism 70 similarly approximate a frictionless contact ofa sphere on a flat surface so that the forces exerted on the drivenbearings 118, 120 are substantially parallel to the X-axis.

[0033] As shown in FIG. 4, the nut mechanism 70 includes a frame 130. Afirst wall 132 of the frame is adjacent the runner bearing 84 andincludes an elongate slot 134 which defines a stationary runner surface136 along which the runner bearing 84 rolls. A second wall 138 of theframe 130 on the opposite side of the nut assembly 78 from the firstwall 132 and adjacent the dummy runner bearing 88 (not visible in FIG.4) similarly includes an elongate slot 140 which defines a dummy runnersurface 142 (best seen in FIG. 5) along which the dummy runner bearing88 rolls. The slot 134 in the wall 132 is slightly larger in width thanthe diameter of the runner bearing 84. Consequently, if forces areapplied to the nut that are greater than the preload provided by thedummy runner 88, the runner bearing 84 will lose contact with the runnersurface 136 and make contact with the upper surface of the slot 134 andthis upper surface will then function as a runner surface.

[0034] As depicted in FIG. 3, the shaft 90 for the dummy runner bearing88 includes a resiliently bendable portion 144 which allows the shaft 90to bend in the YZ plane so that the shaft 90 behaves as a leaf spring.With reference to FIG. 5, the frame 130 includes features which allowthe dummy runner surface 142 to be moved in the Z-axis direction forvarying the degree of bending of the dummy runner bearing shaft 90.Specifically, the frame 130 includes a movable dummy runner 146 whichdefines the dummy runner surface 142. The dummy runner 146 is connectedto the frame side wall 138 by connecting webs 148, and includes oppositeend portions 150 which are disposed in cut-outs 152 formed in the wall138 such that each end portion 150 is between one of the connecting webs148 and an opposite portion of the wall 138 on the other side of thecut-out 152. The end portions 150 are engaged on one side by tensioningscrews 154 which extend in the positive Z-axis direction, and on anopposite side adjacent the connecting webs 148 by tensioning screws 156which extend in the negative Z-axis direction. Thus, advancing thescrews 156 and retracting the screws 154 will cause the dummy runner 146to be moved in the negative Z-axis direction so as to reduce the bendingof the dummy runner bearing shaft 90, and retracting the screws 156 andadvancing the screws 154 will cause the dummy runner 146 to be moved inthe positive Z-axis direction so as to increase the bending of the shaft90. In this manner, it is possible to vary the preload on the dummyrunner bearing 88 which biases the nut 80 in the direction to maintainthe runner bearing 84 in contact with the runner surface 136. However,the dummy runner bearing and dummy runner can be biased in a number ofother manners, if desired.

[0035]FIG. 8 depicts a third preferred embodiment of a nut mechanism inaccordance with the invention. The nut mechanism 160 includes a nut 162which has a resiliently bendable portion 164 which is bendable relativeto the remainder of the nut 162 in the YZ plane. A runner bearing 166 ismounted on the nut 162 and projects outwardly from one side thereofalong an axis 168 that is parallel to the Z-axis. Mounted on the sameside of the nut and spaced from the runner bearing 166 in the Y-axisdirection is a dummy runner bearing 170 which projects outwardly fromthe nut along an axis 172 which is parallel to the Z-axis. Thus, thedummy runner bearing 170 is angularly displaced from the runner bearing166 about the feed screw axis.

[0036] The nut mechanism 160 includes a frame 174 having a runner 176mounted on a wall 178 thereof adjacent the runner bearing 166 and dummyrunner bearing 170. The runner 176 defines a runner surface 180 on oneside thereof and a dummy runner surface 182 on an opposite side thereofspaced from the runner surface in the Y-axis direction. The runnersurface 180 and dummy runner surface 182 extend parallel to each otherin the X-axis direction. The spacing between the runner surface 180 anddummy runner surface 182 in the Y-axis direction is slightly greaterthan the spacing between the outer surface of the runner bearing 166 andthe outer surface of the dummy runner bearing 170 when the resilientportion 164 of the nut 162 is relaxed. Thus, there is a spring forcebetween the dummy runner bearing 170 and the dummy runner surface 182which causes the nut 162 to be rotatably biased to maintain the runnerbearing 166 in contact with the runner surface 180.

[0037] Based on the foregoing description of certain preferredembodiments of the invention, it will be appreciated that the inventionprovides unique feed screw/nut mechanisms having features formaintaining contact between a runner bearing and stationary runnersurface and for minimizing nonaxial force components exerted on acarriage. Although the illustrated embodiments have been described inconsiderable detail, it will be understood that the invention is notlimited to these details. Persons of ordinary skill in the art willreadily comprehend various modifications and substitutions ofequivalents which can be made to the described embodiments, and it isintended that such modifications and substitutions be encompassed withinthe scope of the appended claims. For example, while some of thedescribed embodiments have shown the runner bearing and dummy runnerbearing as being angularly spaced by 180° such that they arediametrically opposite each other, angular spacings other than 180° canbe used. Furthermore, while the nut 162 of FIG. 8 is depicted as havingan integrally formed resilient portion 164, the resilient portion mayalternatively be a separately formed member which is attached to the nut162. Other modifications and substitutions can be made without departingfrom the scope of the following claims.

What is claimed is:
 1. A nut mechanism for translating a carriage along an X-axis, and comprising: an externally threaded feed screw which is rotatable about a fixed screw axis parallel to the X-axis; a nut having an internally threaded bore which threadingly receives the feed screw; a stationary runner adapted to be fixed relative to the feed screw axis, the stationary runner defining a stationary runner surface which extends parallel to the X-axis; a runner bearing attached to the nut and projecting outwardly therefrom along a Y-axis which is perpendicular to the X-axis, the runner bearing engaging the stationary runner surface to prevent rotation of the nut when the feed screw is rotated such that rotation of the feed screw causes the nut to translate along the X-axis and the runner bearing to travel along the stationary runner surface; a dummy runner bearing attached to the nut and projecting outwardly therefrom along an axis which is perpendicular to the X-axis and angularly displaced about the screw axis from the runner bearing; and a dummy runner defining a dummy runner surface which extends parallel to the X-axis and engages the dummy runner bearing; at least one of the dummy runner bearing and dummy runner being biased toward the other so as to rotatably bias the nut in a direction to maintain the runner bearing in contact with the stationary runner surface.
 2. The nut mechanism of claim 1, wherein the dummy runner bearing includes a shaft attached to and projecting outwardly from the nut, at least a portion of the shaft being resiliently bendable so as to bias the dummy runner bearing against the dummy runner surface.
 3. The nut mechanism of claim 1, wherein the dummy runner is movably mounted relative to the dummy runner bearing and is biased toward the dummy runner bearing for rotatably biasing the nut.
 4. The nut mechanism of claim 1, further comprising: a drive bearing attached to the nut and projecting outwardly therefrom, the drive bearing having an outer generally cylindrical drive surface defining an axis which is perpendicular to the X-axis; and a driven bearing which has an outer generally cylindrical driven surface defining an axis and which is adapted to be attached to the carriage such that the axis of the driven surface is perpendicular to both the axis of the drive surface and the X-axis, and such that the driven surface is engaged by the drive surface to form a crossed bearing coupling.
 5. The nut mechanism of claim 4, wherein the drive and driven bearings are freely rotatable about their respective axes such that the crossed bearing coupling is substantially incapable of transmitting forces to the carriage in directions non-parallel to the X-axis.
 6. The nut mechanism of claim 1, wherein the runner bearing and dummy runner bearing are on diametrically opposite sides of the nut with their axes collinear with each other and extending along the Y-axis.
 7. The nut mechanism of claim 1, wherein the runner bearing and dummy runner bearing are freely rotatable about their respective axes so as to roll along the stationary runner surface and dummy runner surface, respectively, when the nut is translated along the X-axis direction.
 8. The nut mechanism of claim 5, wherein the axis of the drive bearing defines a Z-axis which is mutually perpendicular to both the X-axis and the Y-axis.
 9. A nut mechanism for translating a carriage along an X-axis by engagement with an externally threaded feed screw which is rotatable about a fixed screw axis parallel to the X-axis, and comprising: a stationary runner adapted to be fixed relative to the X-axis, the stationary runner defining a stationary runner surface which extends parallel to the X-axis; a nut having an internally threaded bore adapted to threadingly receive the feed screw; a runner bearing attached to the nut and projecting outwardly therefrom, the runner bearing being rotatable about a Y-axis which is perpendicular to the X-axis, the runner bearing engaging the stationary runner surface to prevent rotation of the nut when the feed screw is rotated such that rotation of the feed screw causes the nut to translate along the X-axis and the runner bearing to travel along the stationary runner surface; and a slave carriage connected with the nut and adapted to engage the carriage for transmitting force in the X-axis direction from the nut to the carriage, the slave carriage being connected to the nut so as to be rotatable relative to the nut about the Y-axis.
 10. The nut mechanism of claim 9, further comprising a pair of bearings mounted on opposite sides of the nut and connected to the slave carriage for rotatably connecting the slave carriage to the nut.
 11. The nut mechanism of claim 10, wherein the pair of bearings for mounting the slave carriage to the nut have axes which are collinear with the Y-axis.
 12. The nut mechanism of claim 11, further comprising a dummy runner bearing attached to the nut and projecting outwardly therefrom, the dummy runner bearing being rotatable about an axis which is perpendicular to the X-axis and angularly displaced about the screw axis from the runner bearing; and a dummy runner defining a dummy runner surface which extends parallel to the X-axis and engages the dummy runner bearing; at least one of the dummy runner bearing and dummy runner being biased toward the other so as to rotationally bias the nut in a direction to maintain the runner bearing in contact with the stationary runner surface.
 13. The nut mechanism of claim 12, wherein the dummy runner bearing includes a shaft which is resiliently bendable and is preloaded to urge the dummy runner bearing against the dummy runner surface.
 14. The nut mechanism of claim 12, wherein the dummy runner bearing and the runner bearing are mounted to the nut on diametrically opposite sides thereof and have their respective axes collinearly aligned along the Y-axis.
 15. The nut mechanism of claim 12, further comprising: a first drive bearing attached to a first side of the slave carriage and projecting outwardly therefrom, the first drive bearing having an outer generally cylindrical drive surface defining an axis which is perpendicular to the X-axis; and a first driven bearing which has an outer generally cylindrical driven surface defining an axis and which is adapted to be attached to the carriage such that the axis of the driven surface is perpendicular to both the axis of the drive surface and the X-axis, and such that the driven surface is engaged by the drive surface to form a first crossed bearing coupling.
 16. The nut mechanism of claim 15, further comprising: a second drive bearing having an outer generally cylindrical drive surface defining an axis and attached to the slave carriage on a second side thereof opposite the first drive bearing, the two drive bearings being diametrically opposite each other and symmetrically disposed about the slave carriage; and a second driven bearing having an outer generally cylindrical driven surface defining an axis and adapted to be attached to the carriage with the axis perpendicular to the X-axis and to the axis of the second drive bearing, the second drive and driven bearings being engageable to form a second crossed bearing coupling; whereby the two crossed bearing couplings define a line of action for force transmitted to the carriage which is generally aligned with the screw axis.
 17. The nut mechanism of claim 16, further comprising a connector mounted to the slave carriage and adapted to be connected to the carriage for transmitting force thereto, the connector including first and second spaced-apart walls between which the slave carriage is disposed, the first wall having the first driven bearing mounted thereon and the second wall having the second driven bearing mounted thereon.
 18. The nut mechanism of claim 16, wherein each of the first and second drive bearings comprise a pair of rotatable bearings spaced apart in the direction of the X-axis, the rotatable bearings of each pair having outer generally cylindrical drive surfaces defining axes which are parallel to each other and perpendicular to both the X-axis and the axis of the respective driven bearing, and wherein each driven bearing is disposed between the rotatable bearings of the respective drive bearing to form a biaxial crossed bearing coupling for moving the carriage in two opposite directions along the X-axis.
 19. The nut mechanism of claim 18, wherein the pairs of rotatable bearings on the slave carriage have their axes oriented parallel to the Y-axis and the driven bearings have their axes oriented parallel to a Z-axis which is mutually perpendicular to both the X-axis and the Y-axis.
 20. The nut mechanism of claim 18, wherein the pairs of rotatable bearings on the slave carriage have their axes oriented parallel to a Z-axis which is mutually perpendicular to both the X-axis and the Y-axis and the driven bearings have their axes oriented parallel to the Y-axis.
 21. A nut mechanism for translating a carriage along an X-axis by engagement with an externally threaded feed screw which is rotatable about a fixed screw axis parallel to the X-axis, and comprising: a frame defining an interior space therein and having an opening for passage of the feed screw into the interior space; a nut disposed in the interior space and having an internally threaded bore adapted to threadingly receive the feed screw, the nut including a portion which is resiliently bendable relative to the remainder of the nut in a plane perpendicular to the X-axis; a runner bearing attached to the nut and projecting outwardly therefrom along a first axis which is perpendicular to the X-axis, the runner bearing engaging a stationary runner surface to prevent rotation of the nut when the feed screw is rotated such that rotation of the feed screw causes the nut to translate along the X-axis and the runner bearing to travel along the stationary runner surface; and a dummy runner bearing attached to the resiliently bendable portion of the nut and projecting outwardly therefrom along a second axis which is parallel to the first axis of the runner bearing and spaced apart therefrom, the dummy runner bearing engaging a dummy runner surface; the resiliently bendable portion of the nut preloading the dummy runner bearing against the dummy runner surface so as to rotatably bias the nut in a direction to maintain the runner bearing in contact with the stationary runner surface.
 22. The nut mechanism of claim 21, wherein the runner bearing and dummy runner bearing are mounted on a first side of the nut, and further comprising a pair of rotatable drive bearings mounted on an opposite second side of the nut, the drive bearings having axes which are spaced apart from and parallel to each other and perpendicular to the X-axis.
 23. The nut mechanism of claim 22, further comprising a rotatable driven bearing adapted to be connected to the carriage with an axis of the driven bearing perpendicular to the X-axis and to the axes of the drive bearings, the driven bearing being disposed between the drive bearings so as to form a crossed bearing coupling.
 24. The nut mechanism of claim 23, wherein the frame includes an elongate slot parallel to the X-axis through which the driven bearing extends.
 25. A nut mechanism for translating a carriage along an X-axis by engagement with an externally threaded feed screw which is rotatable about a fixed screw axis parallel to the X-axis, and comprising: a stationary runner adapted to be fixed relative to the X-axis, the stationary runner defining a stationary runner surface which extends parallel to the X-axis; a nut having an internally threaded bore adapted to threadingly receive the feed screw; a runner bearing attached to the nut and projecting outwardly therefrom along a Y-axis which is perpendicular to the X-axis, the runner bearing engaging the stationary runner surface to prevent rotation of the nut when the feed screw is rotated such that rotation of the feed screw causes the nut to translate along the X-axis and the runner bearing to travel along the stationary runner surface; a drive bearing attached to the nut and projecting outwardly therefrom, the drive bearing having an outer generally cylindrical drive surface defining an axis which is perpendicular to the X-axis; and a driven bearing which has an outer generally cylindrical driven surface defining an axis and which is adapted to be attached to the carriage such that the axis of the driven surface is perpendicular to both the axis of the drive surface and the X-axis, and such that the driven surface is engaged by the drive surface to form a crossed bearing coupling.
 26. The nut mechanism of claim 25, wherein the drive and driven bearings are freely rotatable about their axes.
 27. The nut mechanism of claim 25, further comprising a second rotatable drive bearing having an outer generally cylindrical drive surface defining an axis, the two drive bearings being mounted side-by-side on the nut with their axes spaced apart in the X-axis direction and parallel to each other, the driven bearing being disposed between the drive bearings.
 28. A nut mechanism for translating a carriage along an X-axis by engagement with an externally threaded feed screw which is rotatable about a fixed screw axis parallel to the X-axis, and comprising: a nut having an internally threaded bore adapted to threadingly receive the feed screw; a stationary runner fixed relative to the screw axis, opposite sides of the stationary runner respectively defining a stationary runner surface which extends parallel to the X-axis and a dummy runner surface which is spaced apart from and parallel to the stationary runner surface; a runner bearing attached to the nut and projecting outwardly therefrom and being rotatable about a first axis which is perpendicular to the X-axis, the runner bearing engaging the stationary runner surface to prevent rotation of the nut when the feed screw is rotated such that rotation of the feed screw causes the nut to translate along the X-axis and the runner bearing to travel along the stationary runner surface; and a dummy runner bearing attached to the nut and projecting outwardly therefrom and being rotatable about a second axis which is parallel to the first axis of the runner bearing and spaced apart therefrom, the dummy runner bearing engaging the dummy runner surface.
 29. The nut mechanism of claim 28, wherein the nut includes a resiliently bendable portion, the dummy runner bearing being attached to the resiliently bendable portion, and the resiliently bendable portion being preloaded to bias the dummy runner bearing against the dummy runner surface so as to rotatably bias the nut in a direction to maintain the runner bearing in contact with the stationary runner surface. 